1. Field of the Invention
The present invention relates to a signal processor for converting a signal, and more specifically, to a signal processor for converting a signal that performs 2R (re-amplifying, re-shaping) regeneration while converting a return to zero (RZ) signal into a non-return to zero (NRZ) signal.
2. Discussion of Related Art
In general, an optical signal for use in an optical communication system is classified into an RZ signal and an NRZ signal.
The NRZ signal is obtained by modulating a continuous wave (CW). Therefore, it is rather simply generated relative to the RZ signal and less sensitive to a timing jitter than the RZ signal. The bandwidth of an optical receiver for the NRZ signal may be smaller than a rate of the RZ signal. This is because electronic apparatuses should detect change of one bit signal from “1” to “0” for the RZ signal. However, when the NRZ signal is used in a wavelength division multiplexing, signals having different wavelengths are more often overlapped in time than RZ signal, which leads to a nonlinear disturbance.
Further, the RZ signal generally has good sensitivity at a receiving unit, so that it is less restricted by intensity of a signal. In addition, compared with the NRZ signal, the RZ signal is less overlapped in time, thus resulting in small nonlinear phenomena.
To increase a signal transmission rate in the recent communication system, the signal rate is increased up to more than 40 Gbit/s or a wavelength division multiplexing (such as DWDM) is used. However, in this case, there are various problems in that the signal intensity varies across an optical fiber, and that the system is affected a lot by dispersion characteristics, a transmission distance and nonlinear characteristics of the optical fiber. Therefore, all details should be considered, such as which system characteristics and type of signal format should be employed, which characteristics the employed signal should have.
In this respect, various methods are presented that converts the RZ signal into the NRZ signal.
B. Mikkelsen et al. disclose that an RZ signal of 40 Gbit/s is converted into an NRZ signal using a Michelson interferometer as a wavelength converter. When an optical signal having a short pulse width is incident on the Michelson interferometer, carriers of a semiconductor optical amplifier are reduced as light is amplified, while recovered as optical signals pass through the semiconductor optical amplifier. In other words, a wavelength-converted signal is obtained using interferometer characteristics depending on a refractive index variation of the semiconductor optical amplifier. However, for example, when a series of RZ signals of 10 GHz having a short pulse width is incident on the wavelength converter, as long as a special apparatus is not provided, this method will generate a phase recovery speed of the semiconductor optical amplifier slower than a pulse gap of a series of the RZ signals. Therefore, peaks of the incident RZ signals are flattened, and the resulting NRZ signals are obtained (B. Mikkelsen et al., “40 Gbit/s All-Optical Wavelength Converter and RZ to NRZ Format Adapter Realized By Monolithic Integrated Active Mikkelson Interferometer”, Electronics Letters, Vo. 33, pp. 133-144, 1997).
Lei Xu et al. disclose that the incident RZ signals are passed through several delay lines to generate pulse trains having a narrow gap and these signals are injected into the wavelength converter and converted into the NRZ signal. The method has the same principle as that of B. Mikkelsen et al., but with this, the RZ signal is converted into the pulse trains having a narrower gap using several delay lines to obtain the format converted NRZ signals. However, there is a drawback in that the apparatus is complicated due to use of the delay lines and the wavelength converter (Lei Xu et al., “All-Optical Data Format Conversion Between RZ and NRZ Based a Mach-Zehnder Interferometric Wavelength Converter”, IEEE Photonics Technology Letters, Vo. 15, pp. 308-310, 2003).
Sang-Gyu Park et al. present a method in which an input RZ signal is divided into two signals and time-delayed through delay lines, and then, incident on a Mach-Zehnder wavelength converter. With this method, a signal speed can be improved using a time delay, but there is a drawback in that a delay line should be used. In addition, essentially, the Mach-Zehnder wavelength converter does not have a step-like optical transfer curve so that 2R regeneration capability is not good (Sang-Gyu Park et al., “Chirp Consequences Of All-Optical RZ to NRZ Conversion Using Cross-Phase Modulation In An Active Semiconductor Photonic Integrated Circuit”, IEEE Photonics Technology Letters, Vol. 12, pp. 233-235, 2000).